System and method for neighbor direction and relative velocity determination via doppler nulling techniques

ABSTRACT

A system and method for frequency offset determination in a MANET via Doppler nulling techniques is disclosed. In embodiments, a receiving (Rx) node of the network monitors a transmitting (Tx) node of the network, which scans through a range or set of Doppler nulling angles adjusting its transmitting frequency to resolve Doppler frequency offset at each angle, the Doppler frequency shift resulting from the motion of the Tx node relative to the Rx node. The Rx node detects the net frequency shift at each nulling direction and can thereby determine frequency shift points (FSP) indicative of the relative velocity vector between the Tx and Rx nodes. If the set of Doppler nulling angles is known to it, the Rx node can determine frequency shift profiles based on the FSPs, and derive therefrom the relative velocity and angular direction of motion between the Tx and Rx nodes.

BACKGROUND

Mobile Ad-hoc NETworks (MANET; e.g., “mesh networks”) are known in theart as quickly deployable, self-configuring wireless networks with nopre-defined network topology. Each communications node within a MANET ispresumed to be able to move freely. Additionally, each communicationsnode within a MANET may be required to forward (relay) data packettraffic. Data packet routing and delivery within a MANET may depend on anumber of factors including, but not limited to, the number ofcommunications nodes within the network, communications node proximityand mobility, power requirements, network bandwidth, user trafficrequirements, timing requirements, and the like.

MANETs face many challenges due to the limited network awarenessinherent in such highly dynamic, low-infrastructure communicationsystems. Given the broad ranges in variable spaces, the challenges liein making good decisions based on such limited information. For example,in static networks with fixed topologies, protocols can propagateinformation throughout the network to determine the network structure,but in dynamic topologies this information quickly becomes stale andmust be periodically refreshed. It has been suggested that directionalsystems are the future of MANETs, but this future has not as yet beenrealized. In addition to topology factors, fast-moving platforms (e.g.,communications nodes moving relative to each other) experience afrequency Doppler shift (e.g., offset) due to the relative radialvelocity between each set of nodes. This frequency Doppler shiftcomplicates the tradeoffs with respect to, e.g., sensitivity, responsetime, and/or resource utilization that must be made for receptionprocessing. Further, clock frequency errors also contribute to netsignal frequency offsets such that determining relative velocity basedon signal frequency shift is not always straightforward.

SUMMARY

A receiving (Rx) communications node of a multi-node communicationsnetwork is disclosed. In embodiments, the Rx node includes acommunications interface and antenna elements. The Rx node includes acontroller for monitoring a transmitting (Tx) node of the network. TheTx node scans through a range or set of Doppler nulling angles (e.g.,nulling directions) according to a Doppler nulling protocol, adjustingits transmit frequency to resolve at each nulling angle a Dopplerfrequency offset associated with the relative motion (e.g., including avelocity vector and angular direction) between the Tx and Rx nodes.Based on the net frequency shift detected by the Rx node at each Dopplernulling angle, the Rx node determines frequency shift points (FSP),corresponding to each nulling angle and to the associated net frequencyshift, the set of FSPs indicative of a magnitude of the relative radialvelocity between the Tx and Rx nodes.

In some embodiments, the magnitude of the relative radial velocityincludes a maximum and minimum radial velocity between the Tx and Rxnodes, e.g., a relative maximum/minimum for the range of nulling anglesor the absolute maximum/minimum for all possible nulling angles.

In some embodiments, the set of Doppler nulling angles (and hence theset of FSPs) includes at least three elements.

In some embodiments, the set of Doppler nulling angles is known to theRx node, and the controller additionally generates frequency shiftprofiles by plotting the set of FSPs over the set of Doppler nullingangles. The controller further determines, based on the frequency shiftprofiles, additional parameters such as the directional component of therelative radial velocity vector and the angular direction of the Tx noderelative to the Rx node.

In some embodiments, the additional parameters include a phase offset ofthe frequency shift profile.

In some embodiments, the angular direction includes a clock frequencyoffset between the Tx and Rx nodes, which the Rx node determines basedon additional information received from the Tx node.

In some embodiments, the relative radial velocity vector is based in aplatform inertial reference frame specific to the Rx node (e.g., avector of the Tx node's movement relative to the Rx node). The Rx nodemay convert the relative radial velocity vector to a global inertialreference frame, e.g., to account for the motion of both the Tx and Rxnodes relative to the earth.

In some embodiments, the Rx node measures a time differential associatedwith each identified signal, e.g., a time differential in themeasurement of one or more cycles of the received signal, the timedifferential corresponding to the Doppler frequency offset at thecorresponding Doppler nulling angle.

In a further aspect, a method for direction and relative velocitydetermination between transmitting (Tx) and receiving (Rx) nodes in amulti-node communications network is also disclosed. In embodiments, themethod includes identifying, via the Rx node, signals transmitted by aTx node moving relative to the Rx node (e.g., according to a velocityvector and angular direction) and scanning through a range or set ofDoppler nulling angles (e.g., nulling directions). Each signal maycorrespond to an adjustment of the transmitting frequency (e.g.,corresponding to a net frequency shift detected by the Rx node) at aparticular nulling direction to resolve the Doppler frequency offsetassociated with the motion of the Tx node relative to the Rx node. Themethod includes determining, based on the set of identified signals andcorresponding set of net frequency shifts, a set of frequency shiftpoints (FSP), each FSP corresponding to a particular nulling direction,to the net frequency shift at that direction, and to a radial velocityof the Tx node relative to the Rx node. The method includes determining,based on the set of FSPs, a magnitude of the radial velocity vectorbetween the Tx and Rx nodes.

In some embodiments, the method includes determining a maximum andminimum relative velocity between the Tx and Rx nodes (e.g., which maybe absolute or relative maxima and minima).

In some embodiments, the set of nulling angles is known to the Rx node,and the method includes mapping the set of FSPs to the set of nullingangles to generate frequency shift profiles. Based on the frequencyshift profiles, the Rx node determines a directional component of therelative radial velocity vector and the angular direction associatedwith the motion of the Tx node relative to the Rx node.

In some embodiments, the method includes identifying a phase offset ofthe frequency shift profile.

In some embodiments, the method includes determining a clock frequencyoffset between the Tx and Rx nodes, the clock frequency offsetincorporated into the angular direction and determined based onadditional parameters received from the Tx node (e.g., via shiftingscanning and monitoring roles between the Tx and Rx nodes).

In some embodiments, the method includes converting the relative radialvelocity vector from a platform inertial reference frame specific to theRx node to a global reference frame.

In some embodiments, the method includes measuring a time differentialassociated with each identified signal, e.g., a time differential in themeasurement of one or more cycles of the received signal, the timedifferential corresponding to the Doppler frequency offset at thecorresponding Doppler nulling angle.

This Summary is provided solely as an introduction to subject matterthat is fully described in the Detailed Description and Drawings. TheSummary should not be considered to describe essential features nor beused to determine the scope of the Claims. Moreover, it is to beunderstood that both the foregoing Summary and the following DetailedDescription are example and explanatory only and are not necessarilyrestrictive of the subject matter claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. The use of the same reference numbers in different instances inthe description and the figures may indicate similar or identical items.Various embodiments or examples (“examples”) of the present disclosureare disclosed in the following detailed description and the accompanyingdrawings. The drawings are not necessarily to scale. In general,operations of disclosed processes may be performed in an arbitraryorder, unless otherwise provided in the claims. In the drawings:

FIG. 1 is a diagrammatic illustration of a mobile ad hoc network (MANET)and individual nodes thereof according to example embodiments of thisdisclosure;

FIG. 2A is a graphical representation of frequency shift profiles withinthe MANET of FIG. 1,

FIG. 2B is a diagrammatic illustration of varying directional componentsa of the velocity vector of a transmitting node Tx with respect to thegraphical representation of FIG. 2A;

FIG. 3A is a graphical representation of frequency shift profiles withinthe MANET of FIG. 1;

FIG. 3B is a diagrammatic illustration of varying angular directions θof a receiving node Rx with respect to the graphical representation ofFIG. 3A;

and FIGS. 4A through 4C are flow diagrams illustrating a method forDoppler frequency offset determination according to example embodimentsof this disclosure.

DETAILED DESCRIPTION

Before explaining one or more embodiments of the disclosure in detail,it is to be understood that the embodiments are not limited in theirapplication to the details of construction and the arrangement of thecomponents or steps or methodologies set forth in the followingdescription or illustrated in the drawings. In the following detaileddescription of embodiments, numerous specific details may be set forthin order to provide a more thorough understanding of the disclosure.However, it will be apparent to one of ordinary skill in the art havingthe benefit of the instant disclosure that the embodiments disclosedherein may be practiced without some of these specific details. In otherinstances, well-known features may not be described in detail to avoidunnecessarily complicating the instant disclosure.

As used herein a letter following a reference numeral is intended toreference an embodiment of the feature or element that may be similar,but not necessarily identical, to a previously described element orfeature bearing the same reference numeral (e.g., 1, 1a, 1b). Suchshorthand notations are used for purposes of convenience only and shouldnot be construed to limit the disclosure in any way unless expresslystated to the contrary.

Further, unless expressly stated to the contrary, “or” refers to aninclusive or and not to an exclusive or. For example, a condition A or Bis satisfied by any one of the following: A is true (or present) and Bis false (or not present), A is false (or not present) and B is true (orpresent), and both A and B are true (or present).

In addition, use of “a” or “an” may be employed to describe elements andcomponents of embodiments disclosed herein. This is done merely forconvenience and “a” and “an” are intended to include “one” or “at leastone,” and the singular also includes the plural unless it is obviousthat it is meant otherwise.

Finally, as used herein any reference to “one embodiment” or “someembodiments” means that a particular element, feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment disclosed herein. The appearances of thephrase “in some embodiments” in various places in the specification arenot necessarily all referring to the same embodiment, and embodimentsmay include one or more of the features expressly described orinherently present herein, or any combination or sub-combination of twoor more such features, along with any other features which may notnecessarily be expressly described or inherently present in the instantdisclosure.

Broadly speaking, embodiments of the inventive concepts disclosed hereinare directed to a system and method for determining relative velocityvectors, directions, and clock frequency offsets between mutuallydynamic communication nodes of a mobile ad hoc network (MANET) orsimilar multi-node communications network. For example, via the use ofomnidirectional antennas for Doppler null scanning (or, in someembodiments, directional antennas that require directional tracking viaspatial scanning), directional topologies of neighbor nodes in highlydynamic network environments may be determined. Further, if Doppler nullscanning knowledge is common to all nodes, receiving nodes may tune tothe appropriate Doppler frequency shift to maintain full coherentsensitivity.

Referring to FIG. 1, a multi-node communications network 100 isdisclosed. The multi-node communications network 100 may includemultiple communications nodes, e.g., a transmitting (Tx) node 102 and areceiving (Rx) node 104.

In embodiments, the multi-node communications network 100 may includeany multi-node communications network known in the art. For example, themulti-node communications network 100 may include a mobile ad-hocnetwork (MANET) in which the Tx and Rx nodes 102, 104 (as well as everyother communications node within the multi-node communications network)is able to move freely and independently. Similarly, the Tx and Rx nodes102, 104 may include any communications node known in the art which maybe communicatively coupled. In this regard, the Tx and Rx nodes 102, 104may include any communications node known in the art fortransmitting/transceiving data packets. For example, the Tx and Rx nodes102, 104 may include, but are not limited to, radios, mobile phones,smart phones, tablets, smart watches, laptops, and the like. Inembodiments, the Rx node 104 of the multi-node communications network100 may each include, but are not limited to, a respective controller106 (e.g., control processor), memory 108, communication interface 110,and antenna elements 112. (In embodiments, all attributes, capabilities,etc. of the Rx node 104 described below may similarly apply to the Txnode 102, and to any other communication node of the multi-nodecommunication network 100.)

In embodiments, the controller 106 provides processing functionality forat least the Rx node 104 and can include any number of processors,micro-controllers, circuitry, field programmable gate array (FPGA) orother processing systems, and resident or external memory for storingdata, executable code, and other information accessed or generated bythe Rx node 104. The controller 106 can execute one or more softwareprograms embodied in a non-transitory computer readable medium (e.g.,memory 108) that implement techniques described herein. The controller106 is not limited by the materials from which it is formed or theprocessing mechanisms employed therein and, as such, can be implementedvia semiconductor(s) and/or transistors (e.g., using electronicintegrated circuit (IC) components), and so forth.

In embodiments, the memory 108 can be an example of tangible,computer-readable storage medium that provides storage functionality tostore various data and/or program code associated with operation of theRx node 104 and/or controller 106, such as software programs and/or codesegments, or other data to instruct the controller 106, and possiblyother components of the Rx node 104, to perform the functionalitydescribed herein. Thus, the memory 108 can store data, such as a programof instructions for operating the Rx node 104, including its components(e.g., controller 106, communication interface 110, antenna elements112, etc.), and so forth. It should be noted that while a single memory108 is described, a wide variety of types and combinations of memory(e.g., tangible, non-transitory memory) can be employed. The memory 108can be integral with the controller 106, can comprise stand-alonememory, or can be a combination of both. Some examples of the memory 108can include removable and non-removable memory components, such asrandom-access memory (RAM), read-only memory (ROM), flash memory (e.g.,a secure digital (SD) memory card, a mini-SD memory card, and/or amicro-SD memory card), solid-state drive (SSD) memory, magnetic memory,optical memory, universal serial bus (USB) memory devices, hard diskmemory, external memory, and so forth.

In embodiments, the communication interface 110 can be operativelyconfigured to communicate with components of the Rx node 104. Forexample, the communication interface 110 can be configured to retrievedata from the controller 106 or other devices (e.g., the Tx node 102and/or other nodes), transmit data for storage in the memory 108,retrieve data from storage in the memory, and so forth. Thecommunication interface 110 can also be communicatively coupled with thecontroller 106 to facilitate data transfer between components of the Rxnode 104 and the controller 106. It should be noted that while thecommunication interface 110 is described as a component of the Rx node104, one or more components of the communication interface 110 can beimplemented as external components communicatively coupled to the Rxnode 104 via a wired and/or wireless connection. The Rx node 104 canalso include and/or connect to one or more input/output (I/O) devices.In embodiments, the communication interface 110 includes or is coupledto a transmitter, receiver, transceiver, physical connection interface,or any combination thereof.

It is contemplated herein that the communication interface 110 of the Rxnode 104 may be configured to communicatively couple to additionalcommunication interfaces 110 of additional communications nodes (e.g.,the Tx node 102) of the multi-node communications network 100 using anywireless communication techniques known in the art including, but notlimited to, GSM, GPRS, CDMA, EV-DO, EDGE, WiMAX, 3G, 4G, 4G LTE, 5G,WiFi protocols, RF, LoRa, and the like.

In embodiments, the antenna elements 112 may include directional oromnidirectional antenna elements capable of being steered or otherwisedirected (e.g., via the communications interface 110) for spatialscanning in a full 360-degree arc (114) relative to the Rx node 104.

In embodiments, the Tx node 102 and Rx node 104 may both be moving in anarbitrary direction at an arbitrary speed, and may similarly be movingrelative to each other. For example, the Tx node 102 may be movingrelative to the Rx node 104 according to a velocity vector 116, at arelative velocity V_(Tx) and a relative angular direction (an angle αrelative to an arbitrary direction 118 (e.g., due east); θ may be theangular direction of the Rx node relative to due east.

In embodiments, the Tx node 102 may implement a Doppler nullingprotocol. For example, the Tx node 102 may adjust its transmit frequencyto counter the Doppler frequency offset such that there is no netfrequency offset (e.g., “Doppler null”) in a Doppler nulling direction120 (e.g., at an angle ϕ relative to the arbitrary direction 118). Thetransmitting waveform (e.g., the communications interface 110 of the Txnode 102) may be informed by the platform (e.g., the controller 106) ofits velocity vector and orientation (e.g., a, V_(T)) and may adjust itstransmitting frequency to remove the Doppler frequency shift at eachDoppler nulling direction 120 and angle ϕ.

In embodiments, even if the Doppler nulling protocol is not known to theRx node 104, the Rx node may observe (e.g., monitor, measure) the netfrequency offset as the Tx node 102 covers (e.g., steers to, orients to,directs antenna elements 112 to) a range of Doppler nulling directions120 (e.g., relative to the arbitrary direction 118, each Doppler nullingdirection 120 having a corresponding Doppler nulling angle ϕ).Accordingly, the Rx node 104 may determine the magnitude of theparameter A of the velocity vector {right arrow over (V′_(T) )} of theTx node 102, to the degree that the Tx node covers both extremes (e.g.,achieves both a minimum and a maximum velocity relative to the Rx node)such that

$A = {\frac{f}{c}{❘\overset{\rightarrow}{V_{T}^{\prime}}❘}}$

where f is the transmitting frequency of the Tx node and c is the speedof light. For example, each frequency shift point (FSP) detected by theRx node 104 at a given Doppler nulling direction 120 may correspond to avelocity vector of the Tx node 102 relative to the Rx node. As notedabove, and as described in greater detail below, the magnitude parameterA may incorporate a maximum and minimum relative velocity. If, however,the range of Doppler nulling angles ϕ is insufficiently broad, themagnitude parameter A may only include relative maxima and minima forthat limited range of Doppler nulling angles (e.g., as opposed to thefull 360 degrees of possible Doppler nulling angles; see, for example,FIGS. 2A-3B below).

In some embodiments, the Doppler nulling protocol and set of Dopplernulling directions 120 (and corresponding angles ϕ) may be known to theRx node 104 and common to all other nodes of the multi-nodecommunications network 100. For example, the Tx node 102 may perform theDoppler nulling protocol by pointing a Doppler null in each Dopplernulling direction 120 and angle ϕ of the set or range of directions asdescribed above. The Rx node 104 may monitor the Tx node 102 as theDoppler nulling protocol is performed and may therefore determine, andresolve, the net Doppler frequency shift for each Doppler nullingdirection 120 and angle ϕ.

In embodiments, although both the Tx and Rx nodes 102, 104 may be movingrelative to the arbitrary direction 118, monitoring of the Dopplernulling protocol by the Rx node 104 may be performed and presented inthe inertial reference frame of the Rx node 104 (e.g., in terms of themovement of the Tx node 102 relative to the Rx node 104) to eliminatethe need for additional vector variables corresponding to the Rx node.For example, the velocity vector of the Tx node 102 in a globalreference frame may be shifted according to the velocity vector of theRx node 104, e.g.:

{right arrow over (V′ _(T))}={right arrow over (V _(T))}−{right arrowover (V _(R))}

where {right arrow over (V′_(T))} is the velocity vector of the Tx nodein the inertial reference frame of the Rx node and {right arrow over(V_(T))}, {right arrow over (V_(R))} are respectively the velocityvectors of the Tx node and the Rx node in the Earth reference frame. Inembodiments, either or both of the Tx node 102 and Rx node 104 mayaccordingly compensate for their own velocity vectors relative to theEarth and convert any relevant velocity vectors and relative velocitydistributions into a global reference frame, e.g., for distributionthroughout the multi-node communications network 100. In addition, whilethe representation of the relative motion between the Tx and Rx nodes102, 104 is here presented in two dimensions, the relative motion (and,e.g., any associated velocity vectors, angular directions, Dopplernulling directions, and other parameters) may be presented in threedimensions with the addition of vertical/z-axis components.

Referring now to FIGS. 2A and 2B, the graph 200 and multi-nodecommunication network 100 are respectively shown. The graph 200 may plotfrequency shift profiles for varying directional components (α, FIG. 2B)of the velocity vector of the Tx node (102, FIG. 2B) relative to the Rxnode (104, FIG. 2B) for multiple Doppler nulling directions (120,FIG. 1) and angles ϕ) (e.g., relative to the arbitrary direction (118,FIG. 2B)) and velocity V_(Tx) of the Tx node. In the interest ofclarity, the graph 200 and other plots of frequency shift profilesprovided below may be scaled by c/f to eliminate the ratio f/c (where,as noted above, f is the transmitting frequency of the Tx node 102 and cis the speed of light).

In embodiments, the Rx node 104 may repeat the net Doppler frequencyshift determination and resolution process for multiple Doppler nullingdirections 120 and angles ϕ of the Tx node 102 (e.g., chosen at randomor according to predetermined or preprogrammed protocol). For example,the Tx node 102 may scan through at least three Doppler nullingdirections (202 a-c, FIG. 2B)/angles ϕ and map, via the correspondingfrequency shift points, the distribution of the dependent Dopplerfrequency shift for each Doppler nulling direction and angle ϕ. Thegraph 200 may plot frequency shift profiles for varying directionalcomponents a relative to the arbitrary direction 118 assuming theangular direction θ=0 (e.g., consistent with an Rx node 104 moving dueeast) and velocity V_(Tx) of the Tx node 102=1500 m/s. As it is wellknown that the Doppler frequency shift is a sinusoidal distributionrelative to the angle ϕ of the Doppler nulling directions 202 a-c,measurements at multiple Doppler nulling directions of the Tx node 102by the Rx node 104 may generate frequency shift points (204 a-c, FIG.2A) to which a frequency shift profile 206 may be mapped as a sinusoidalcurve showing the distribution of relative velocity between the Tx andRx nodes 102, 104 through the full range of Doppler nulling angles ϕ(e.g., assuming the maximum and minimum relative velocities areincluded).

In embodiments, the amplitude of the frequency shift profile 206 maycorrespond to the velocity of the Tx node 102 relative to the Rx node104. For example, even if the Doppler nulling protocol is not known tothe Rx node 104, a magnitude parameter A of the velocity vector {rightarrow over (V′_(T))} of the Tx node 102 (e.g., in the reference frame ofthe Rx node) may still be determined, e.g., between a minimum relativevelocity 208 (e.g., 0 m/s) and a maximum relative velocity 210 (e.g.,3000 m/s, or consistent with Tx and Rx nodes traveling in opposingdirections (α=180°, consistent with a Tx node traveling due west (212)and the phase-offset frequency shift profile 214).

In embodiments, as a varies the frequency shift profiles 214, 216, 218may present as phase-offset versions of the frequency shift profile 206(e.g., with similarly offset maximum and minimum relative velocities).For example (in addition to the frequency shift profile 214 notedabove), the frequency shift profile 216 may correspond to α=90°,consistent with a Tx node traveling due north (220) and the frequencyshift profile 218 may correspond to α=−90°, consistent with a Tx nodetraveling due south (222).

In embodiments, the frequency shift profiles 206, 214, 216, 218 mayallow the Rx node 104 to derive parameters in addition to the magnitudeparameter A of the velocity vector {right arrow over (V′_(T))} of the Txnode 102. For example, the true Doppler frequency shift due to therelative radial velocity between the Tx and Rx nodes 102, 104 may be, asseen by the Rx node:

${\Delta f_{Doppler}^{\prime}} = {\frac{f}{c}{❘\overset{\rightarrow}{V_{T}^{\prime}}❘}{\cos\left( {\theta - \alpha} \right)}}$

and the Tx node 102 may, per the Doppler nulling protocol, adjust thetransmitting frequency f due to its velocity projection at the Dopplernulling angle ϕ such that:

${\Delta f_{\Pr}} = {{- \frac{f}{c}}{❘\overset{\rightarrow}{V_{T}^{\prime}}❘}{\cos\left( {\varphi - \alpha} \right)}}$

and the net Doppler frequency shift, also accounting for clock frequencyoffset Δf_(clock), may therefore be:

${\Delta f_{net}} = {{\frac{f}{c}{{❘\overset{\rightarrow}{V_{T}^{\prime}}❘}\left\lbrack {{\cos\left( {\theta - \alpha} \right)} - {\cos\left( {\varphi - \alpha} \right)}} \right\rbrack}} + {\Delta f_{clock}}}$

assuming, for example, that the velocity vector and direction changeslowly relative to periodic measurements of Δf_(net). It should be notedthat Δf_(net) as presented above represents a net frequency offset fromnominal incorporating f/C (compare, e.g., FIGS. 2A-B and accompanyingtext above). Under these conditions, from the perspective of the Rx node104 the parameters α, T_(x), and θ may be taken as constants, and thenet frequency offset Δf_(net) may also be expressed as:

Δf _(net) =A cos(φ+B)+C

where the constant parameters A, B, and C may be determined via at leastthree measurements of a Doppler nulling angle ϕ As noted above,

$A = {\frac{f}{c}{❘\overset{\rightarrow}{V_{T}^{\prime}}❘}}$

while also

B = π − α and$C = {{\frac{f}{c}{❘\overset{\rightarrow}{V_{T}^{\prime}}❘}{\cos\left( {\theta - \alpha} \right)}} + {\Delta f_{clock}}}$

where, as noted above, A may correspond to the magnitude of the velocityvector of the Tx node 102 relative to the Rx node 104. Similarly, B maycorrespond to the directional component α of the velocity vector and Cto the angular direction θ of the Rx node 104.

In embodiments, once the parameters A, B, and C are determined, theparameters α, V′_(T), θ, may be derived therefrom as can be seen above.For example, when the clock frequency offset Δf_(clock) is zero it isstraightforward to derive θ from C above. However, when the clockfrequency offset Δf_(clock) is nonzero, the Rx node 104 may determineΔf_(clock) by exchanging information with the Tx node 102. For example,the Rx and Tx nodes 104, 102 may switch roles: the Rx node 104 mayperform the Doppler nulling protocol for various Doppler nullingdirections 120 and angles ϕ while the Tx node 102 monitors the Dopplernulling protocol to resolve the net Doppler frequency shift for θ′=θ+π(and Δf′_(clock)=−Δf_(clock)). The Tx node 102 may share thisinformation with the Rx node 104, which may merge information from bothdirections to determine θ and Δf_(clock).

Referring now to FIGS. 3A and 3B, the graph 300 and multi-nodecommunication network 100 a may be implemented and may functionsimilarly to the graph 200 and multi-node communication network 100 ofFIGS. 2A and 2B, except that the graph 300 and multi-node communicationnetwork 100 a may reflect a consistent zero directional component α(e.g., a Tx node (102, FIG. 3B) moving in or parallel to the arbitrarydirection (118, FIG. 3B, e.g., due east)) and variable angulardirections θ of the Rx node (104, 104 a-c, FIG. 3B) relative to the Txnode.

In embodiments, the frequency profiles (302, 304, 306, 308; FIG. 3A) mayrespectively be associated with θ=0° (e.g., consistent with the Rx node104 lying directly in the path of the Tx node 102); θ=90° (Rx node 104a); θ=180° (Rx node 104 b, consistent with the Tx node moving in theopposing direction from the Rx node (e.g., an Rx node moving due west));and θ=−95° (Rx node 104 c). Referring in particular to FIG. 3A, thefrequency profiles 302-308 may be shifted in amplitude (rather than inphase, as shown by the graph 200 of FIG. 2A) such that the Dopplerfrequency shift varies only in magnitude (e.g., relative maximum andminimum velocities). It may be noted that the frequency shift profile304 (θ=90°) appears identical to the frequency shift profile associatedwith θ=−90° (Rx node 104 d), where both angular directions θ areperpendicular to the velocity vector of the Tx node 102 (directionalcomponent α) but mutually opposed. If, for example, an Rx node 104 a,104 d communication node enters the multi-node communication network 100a at such a position and velocity, a one-time determination may have tobe made by other means (e.g., or by waiting for a change in Rx nodevelocity or in θ to precisely determine θ (e.g.,)+90°/−90°, after whichdetermination the precise θ can be tracked without ambiguity.

In some embodiments, the Rx node 104, 104 a-c may assess and determineDoppler effects due to the relative motion of the Tx node 102 bymeasuring time differential points (TDP) rather than FSPs. For example,a signal transmitted at 1 kHz by the Tx node 102 may be subject to 10 Hzof Doppler frequency shift. This one-percent (1%) change in frequencymay be alternatively expressed as a differential of one percent in thetime required to measure a cycle of the transmitted signal (or, e.g.,any arbitrary number of cycles). The Doppler effect may be precisely andequivalently characterized in either the frequency domain or the timedomain. For example, the graphs 200, 300 of FIGS. 2A and 3A, which plotthe velocity vector of the Tx node 102 relative to the Rx node 104, 104a-c (y-axis) against the Doppler nulling angle ϕ, may remain consistentbetween the frequency domain and the time domain, with the exceptionthat each FSP (204 a-c, FIG. 2A) corresponds to a measured timedifferential at a given Doppler nulling angle ϕ(e.g., to a TDP) ratherthan to a measured frequency shift at that nulling angle.

In some embodiments, due to the nature of the transmitted signal (or,e.g., other conditions) it may be easier or more advantageous for the Rxnode 104 to determine the Doppler shift in the time domain rather thanin the frequency domain. For example, when the signal transmitted by theTx node 102 at a given Doppler nulling direction (202 a-c, FIG. 2B)consists of a series of short pulses and a long pulse repetitioninterval (e.g., as opposed to, e.g., a continuous short-duration pulse),the Rx node 104 may instead determine the Doppler shift to be resolvedby measuring the time differential between received cycles of thetransmitted signal and generating time differential profiles based oneach determined set of TDPs. As the resulting time differential profilesplot the relative velocity vector of the Tx node 102 over a set ofDoppler nulling angles ϕ similarly to the frequency shift profile graphs200, 300, of FIGS. 2A and 3A, the same information can be determined bythe Rx node 104.

FIGS. 4A-C—Method

Referring now to FIG. 4A, the method 400 may be implemented by themulti-node communications networks 100, 100 a and may include thefollowing steps.

At a step 402, a receiving (Rx) node of the multi-node communicationsnetwork monitors a transmitting (Tx) node of the network to identifysignals transmitted by the Tx node through a range of Doppler nullingangles (e.g., or a set of discrete Doppler nulling angles), the signalsincluding adjustments to the transmitting frequency to counter Dopplerfrequency offset at each Doppler nulling angle. For example, the Tx nodemay be moving relative to the Rx node according to a velocity vector andan angular direction. Each identified signal may correspond to aparticular Tx frequency adjustment (e.g., a net frequency shift detectedby the Rx node) at a particular Doppler nulling angle to resolve aDoppler frequency offset at that angle.

At a step 404, a controller of the Rx node determines, based on themonitoring and identified signals, a set (e.g., three or more) offrequency shift points (FSP), where each FSP corresponds to a netfrequency shift of the signal. For example, each FSP may correspond tothe Tx node (e.g., aware of its velocity vector and platformorientation) scanning in a Doppler nulling direction and adjusting itstransmit frequency to resolve the Doppler offset at the correspondingDoppler nulling angle ϕ according to a nulling protocol, resulting inthe net frequency shift detected by the Rx node. In some embodiments,the Rx node measures the net frequency shift in the time domain ratherthan in the frequency domain. For example, the Rx node may measure atime differential associated with a received cycle or cycles of theidentified signal, the time differential corresponding to the netfrequency shift at the corresponding Doppler nulling angle.

At a step 406, the controller determines, based on the plurality offrequency shift points, a magnitude of the relative velocity vectorbetween the Tx and Rx nodes (e.g., in the reference frame of the Rxnode). For example, from the magnitude of the velocity can be derived amaximum and minimum relative velocity with respect to the range ofDoppler nulling angles ϕ.

In some embodiments, the range or set of Doppler nulling angles ϕ may beknown to all nodes of the multi-node communications network (e.g.,including the Rx node) and the method 400 may include the additionalsteps 408 and 410.

At the step 408, the Rx node maps the determined FSPs to a frequencyshift profile corresponding to a distribution (e.g., a sinusoidal curve)of the ϕ-dependent net frequency shift over all possible Doppler nullingangles ϕ. In some embodiments, the controller further determines a phaseoffset of the frequency shift profile.

At the step 410, the controller determines, based on the frequency shiftprofile, a velocity V′_(T) and a directional component α of the velocityvector (e.g., of the Tx node 102 relative to an arbitrary direction) andthe angular direction θ (e.g., of the Rx node relative to the arbitrarydirection).

Referring also to FIG. 4B, the method 400 may include an additional step412. At the step 412, the angular direction θ incorporates a clockfrequency offset between the Tx and Rx nodes, which the Rx nodedetermines based on additional information received from the Tx node.

Referring now to FIG. 4C, the method 400 may include an additional step414. At the step 414, the velocity vector may be in an inertialreference frame specific to the Rx node. For example, the Rx node mayconvert the velocity vector from its own platform reference frame to aglobal reference frame.

CONCLUSION

It is to be understood that embodiments of the methods disclosed hereinmay include one or more of the steps described herein. Further, suchsteps may be carried out in any desired order and two or more of thesteps may be carried out simultaneously with one another. Two or more ofthe steps disclosed herein may be combined in a single step, and in someembodiments, one or more of the steps may be carried out as two or moresub-steps. Further, other steps or sub-steps may be carried in additionto, or as substitutes to one or more of the steps disclosed herein.

Although inventive concepts have been described with reference to theembodiments illustrated in the attached drawing figures, equivalents maybe employed and substitutions made herein without departing from thescope of the claims. Components illustrated and described herein aremerely examples of a system/device and components that may be used toimplement embodiments of the inventive concepts and may be replaced withother devices and components without departing from the scope of theclaims. Furthermore, any dimensions, degrees, and/or numerical rangesprovided herein are to be understood as non-limiting examples unlessotherwise specified in the claims.

We claim:
 1. A communications node of a multi-node communicationsnetwork, comprising: a communications interface including at least oneantenna element; a controller operatively coupled to the communicationsinterface, the controller including one or more processors andconfigured to: identify a plurality of signals transmitted by a neighbornode of the communications node, the plurality of signals correspondingto a set of Doppler nulling angles traversed by the neighbor node and toat least one frequency adjustment of the neighbor node to resolve aDoppler frequency offset at the corresponding Doppler nulling angle, theneighbor node moving relative to the communications node according toone or more of a velocity vector and an angular direction; determine,based on the plurality of identified signals, a plurality of frequencyshift points (FSP), each FSP corresponding to the Doppler frequencyoffset at the corresponding Doppler nulling angle and associated with arelative radial velocity between the communications and neighbor nodes;and determine, based on the plurality of FSPs, a magnitude of thevelocity vector.
 2. The communications node of claim 1, wherein themagnitude of the velocity vector includes one or more of a maximumrelative velocity and a minimum relative velocity between thecommunications and neighbor nodes.
 3. The communications node of claim1, wherein the plurality of FSPs includes at least three FSPs.
 4. Thecommunications node of claim 1, wherein: the set of Doppler nullingangles is known to the neighbor and communications nodes; and thecontroller is configured to: generating at least one frequency shiftprofile plotting the plurality of FSPs over the set of Doppler nullingangles; and determining, based on the at least one frequency shiftprofile, one or more parameters selected from a group including: adirectional component of the velocity vector; and the angular direction.5. The communications node of claim 4, wherein the one or moreparameters are associated with a phase offset of the at least onefrequency shift profile.
 6. The communications node of claim 4, wherein:the angular direction includes a clock frequency offset; and thecontroller is configured to determine the clock frequency offset basedon additional parameters received from the neighbor node.
 7. Thecommunications node of claim 1, wherein: the velocity vector isassociated with a platform reference frame of the communications node;and the controller is configured to convert the velocity vector from theplatform reference frame to a global reference frame.
 8. Thecommunications node of claim 1, wherein: the controller is configured todetermine each FSP by measuring a time differential associated with eachidentified signal, the time differential corresponding to the Dopplerfrequency offset at the corresponding Doppler nulling angle.
 9. A methodfor neighbor-node direction and relative velocity determination in amulti-node communications network, the method comprising: identifying,via a receiving (Rx) node of the multi-node communications network, aplurality of signals transmitted by a transmitting (Tx) node of themulti-node communications network the plurality of signals correspondingto a set of Doppler nulling angles traversed by the Tx node and to atleast one frequency adjustment of the Tx node to resolve a Dopplerfrequency offset at the corresponding Doppler nulling angle, the Tx nodemoving relative to the Rx node according to one or more of a velocityvector and an angular direction; determining via the Rx node, based onthe plurality of identified signals, a plurality of frequency shiftpoints (FSP), each FSP corresponding to a Doppler frequency shift at thecorresponding Doppler nulling angle and to a relative radial velocitybetween the Tx and Rx nodes; and determining, based on the plurality ofFSPs, a magnitude of the velocity vector.
 10. The method of claim 9,wherein determining, based on the plurality of frequency shift points, amagnitude of the velocity vector includes: determining one or more of amaximum relative velocity and a minimum relative velocity of the Txnode.
 11. The method of claim 9, wherein the at least one Dopplernulling angle is known to the Tx and Rx nodes, further comprising:generating at least one frequency shift profile plotting the pluralityof FSPs over the set of Doppler nulling angles; and determining, basedon the at least one frequency shift profile, one or more parametersselected from a group including: a directional component of the velocityvector; and the angular direction.
 12. The method of claim 11, whereingenerating at least one frequency shift profile plotting the pluralityof FSPs over the set of Doppler nulling angles includes: identifying aphase offset associated with the at least one frequency shift profile.13. The method of claim 11, wherein the angular direction includes aclock frequency offset, further comprising: determining the clockfrequency offset based on additional parameters received from the Txnode.
 14. The method of claim 9, wherein the velocity vector isassociated with a platform reference frame of the Rx node, furthercomprising: converting, via the Rx node, the velocity vector from theplatform reference frame to a global reference frame.
 15. The method ofclaim 9, wherein determining via the Rx node, based on the plurality ofidentified signals, a plurality of frequency shift points (FSP)includes: measuring a time differential associated with each identifiedsignal, each time differential corresponding to the Doppler frequencyoffset at the corresponding Doppler nulling angle.